BA138(N,GAMMA)BA139 REACTION AND EVIDENCE FOR DIRECT CAPTURE

The γ radiation emitted following thermal-neutron capture in Ba138 has been studied both in singles and coincidence with 9-cm3 and 20-cm3 Ge(Li) detectors and a 9-cm3 Ge(Li)-NaI detector combination at the Brookhaven research reactors. γ rays of the following energies (in keV) and relative intensities (given in parentheses) were assigned to Ba139: 295 (<1), 352 (<1), 454.67 (27), 627.26 (100), 666.5 (1.5), 1047.4 (3), 1076 (<1), 1105 (2), 1420.1 (5), 1500 (<1), 1558.0 (2.5), 1854.0 (3), 1952.3 (3), 2242.0 (5), 2522.5 (2.5), 2537.0 (3), 2567 (3), 2594.1 (8), 3432 (<1), 3641.4 (20), and 4096.1 (60). All but one of these transitions were incorporated in a level scheme for the Ba139 nucleus with levels at the following energies (in keV): 627.26±0.10, 1081.9±0.2, 1292.6±1.0, 1420.1±1.0, 1952.3±1.0, 2129.3±1.2, 2156.4±2.0, 2186.4±1.9, and 2481.4±1.2. The level at 1292 keV which is excited in the Ba138 (d,p) reaction is shown to be a doublet of levels at 1284.0 and 1292.6 keV, of which only the 1292.6-keV member is observed to be populated in the Ba138(n,γ) reaction. The neutron separation energy was found to be 4723.4±0.7 keV. The strengths of the primary γ rays and proton groups in the (n,γ) and (d,p) reactions, respectively, have been compared, and a strong correlation, with a correlation coefficient of 0.95, was observed. This correlation is discussed in terms of the common unique parent assumption of Lane and Wilkinson. It is seen to be consistent with this assumption and yields evidence for direct capture. It is shown that the theory of radiative capture of Lane and Lynn predicts a cross section for direct capture in Ba138 which is comparable to the observed total thermal-neutron-capture cross section σth=0.35±0.15 b. © 1969 The American Physical Society.